One conventional application for a solenoid is in the fuel system for a diesel engine, as in a diesel powered truck, automobile, or other vehicle. The solenoid plunger is connected to the fuel pump so that the plunger is at its maximum extension when the solenoid is de-energized. For this operating condition, the fuel supply to the diesel engine is cut off. When the engine is to be started, the solenoid is energized to retract its plunger and thus allow the fuel pump to supply fuel to the engine. Usually, a dual coil solenoid is employed, incorporating a pulling coil and a holding coil. Both coils are energized to retract the solenoid plunger. When the retraction movement of the plunger is completed, the pulling coil is de-energized but the holding coil remains energized to keep the plunger retracted. To shut off the engine, the holding coil is de-energized; a return spring in the solenoid restores the plunger to its initial extended position. Similar solenoid uses occur with stationary engines and in other applications.
In an arrangement of this kind, an electrical failure in the solenoid or in its electrical circuits may preclude energization of the solenoid coils, particularly the pulling coil. In these circumstances, with many solenoids it is essentially impossible to retract the solenoid plunger to allow operation of the engine long enough to get the truck or other vehicle to a location where repairs can be conveniently effected. Thus, it may be necessary to tow the vehicle to a repair facility. In industrial applications, a similar electrical failure may cause a costly interruption in a critical industrial process.
To meet these difficulties, solenoids have been equipped with mechanisms to enable manual actuation of the solenoid to simulate normal electrical operation. Several different embodiments of manual actuation mechanisms for solenoids are disclosed in Mishler et al U.S. Pat. No. 4,679,017. Each of those manual actuator mechanisms includes some form of retainer means to preclude accidental, unwanted actuation caused by vibration, by tinkerers who don't know what the manual actuator does, or from other incidental sources. Another example of a device equipped with a manual actuator is shown in Arnold West German patent publication No. 26 45 887, directed to a solenoid-operated valve. By incorporation of effective, vibration-proof retainer constructions in such solenoids tends to increase their cost undesirably and may make operation of the manual actuator mechanism awkward, difficult, or difficult to understand.